In a suspension spring for an automobile, further weight reduction is required in order to improve automobile fuel efficiency, and in recent years, springs of steel having tensile strength of 1800 MPa or more has also been realized. However, in the case in which structure of the steel is tempered martensite, since generally the notch sensitivity is increased as the steel is strengthened, there is concern that there will be a deleterious influence on spring properties such as fatigue in corrosive environments, etc., and a steel having both high strength and high ductility, in which cracks hardly develop, is required.
In order to solve such problems, a suspension spring steel in which the toughness is improved and the delayed fracture resistance is increased by using steel in which Ni, Nb, etc., are added to standard steel composition of steel for spring regulated under the Japanese Industrial Standard, has been proposed (Patent Publication 1). In addition, a steel for spring in which the embrittlement due to hydrogen penetrating into the steel is prevented and corrosion fatigue resistance is improved by using steel in which at least one of Ti, V, Nb, Zr, and Hf is added, has been proposed (Patent Publication 2). However, since these additive elements are expensive, there is a drawback in that the cost of the steel is increased.
In addition, a high-strength and high-toughness steel in which primary structure is a tempered martensite structure and to which it is not necessary to add large amounts of Ni, Cr, etc., has been proposed (Patent Publication 3). This steel is obtained by regulating the average particle size and aspect ratio of old austenite grains, size and numbers per observed unit cross section of carbide, etc. However, there is a problem in that the process is complicated and the productivity is reduced, since it is necessary to carry out cold working having true strain of 0.2 or more at 500° C. or less before the quenching process.
Furthermore, a high-strength and high-toughness steel in which it is not necessary to add large amounts of alloying elements and special thermomechanical treatment has been proposed, and the toughness is improved by dispersing and precipitating fine carbide in austenite, and by miniaturizing substructures of martensite (Patent Publication 4). However, it is necessary to accurately control temperature and time of heating during the heating in the quenching process in order to allow undissolved carbide to remain, and there is a problem in that the process control is complicated.
In the meantime, a high-toughness steel plate of bainitic structure using a strengthening method of high-strength steel which does not use a quenching and tempering process, has been proposed (Patent Publication 5). This steel plate is obtained by heating low alloy medium carbon steel (Nb content: 0.005 to 0.2%) to an austenite range and then by isothermally holding at a temperature of Ms point or more (austempering treatment). However, it is described that the tensile strength of the obtained steel plate is at maximum about 1530 MPa and the elongation is 9.0%, and sufficient strength and elongation are not obtained. This reason is that the Patent Publication 5 discloses a technology for a steel plate having a Vickers hardness of about 400 HV, supporting carrying out after processing such as bending, reduction of area, etc., and an austempering processing condition is examined only at a relatively high temperature.
In addition, a steel for spring in which the primary structure is a bainite or martensite structure and hydrogen embrittlement resistance is improved by limiting the content of retained austenite and the aspect ratio of crystal grain, has been proposed (Patent Publication 6). Here, in the Patent Publication 6, increased amount of toughness of the steel and characteristic value of ductility such as elongation, reduction of area, etc., in a tensile test, are not described. In addition, one of the important structural factors for having both high strength and high ductility is that average C content in the retained austenite is high, as describe below. In the Patent Publication 6, the factor is not considered, and it is anticipated that the high strength is obtained, but the high ductility is difficult to realize, even if the amount and the shape of the retained austenite are controlled.
Patent Publication 1 is Japanese Patent Publication No. 3783306. Patent Publication 2 is Japanese Unexamined Patent Application Publication No. 2005-23404. Patent Publication 3 is Japanese Unexamined Patent Application Publication No. 2001-288530. Patent Publication 4 is Japanese Unexamined Patent Application Publication No. 2002-212665. Patent Publication 5 is Japanese Examined Patent Application Publication No. S51-29492. Patent Publication 6 is Japanese Unexamined Patent Application Publication No. 2007-100209.